The human eye (and all eyes, really) are incredibly complex organs that absorb and focus light into electrical signals our brains turn into the sense of sight. The outermost layer of the eye is called the cornea which serves several functions important to vision while also keeping the rest of the eye healthy and safe.
What is the Cornea?
The cornea is the outermost part of the eye and the one that directly interacts with the outside world. The cornea is responsible for focusing light coming into the eye. It is five separate layers that each serve functions critical to maintaining healthy eyes and good vision.
The five layers of the cornea
- Epithelium – The outermost layer of the cornea responsible for keeping foreign particles out of the eye, as well as absorbing oxygen and nutrients from tears.
- Bowman’s Layer – This second layer of the cornea is a thin but strong membrane of collagen which gives the cornea rigidity. If damaged, it can become scarred and contribute to vision loss.
- Stroma – This thickest layer of the cornea, composed mostly of water and proteins, gives the cornea its elasticity. The lack of blood vessels in this layer is what makes the cornea transparent.
- Descemet’s Layer – Similar to Bowman’s layer, the Descemet’s layer is a collagen membrane that works to prevent injury and infection.
- Endothelium – This thinnest part of the cornea is only a single layer of cells and is responsible for expelling excess water absorbed by the stroma, keeping the cornea clear.
What Conditions Can Affect the Cornea?
There are many things that can affect or damage the cornea, from a poke to the eye to genetic disorders. Here are the five most common:
Most people will suffer from some form of allergies in their life. Allergens like pollen, dust, mold, and dander can irritate the eyes and cause symptoms such as redness, itching, burning, watery discharge, hazy vision, puffiness, and the sensation there is something in your eye.
Allergies can usually be treated easily with antihistamines and by avoiding the source allergen. Eye drops can also be helpful to reduce symptoms and relieve irritation.
Dry eye disease is a condition that causes discomfort. It can range from itchiness, burning sensation, excessive watering, and if serious enough, eye pain. Dry eye occurs when eyes do not provide enough tears, or tears that are not of sufficient quality to properly lubricate and protect your eyes. Aside from providing moisture, your tears also protect your eyes from infection, rinse out foreign matter, and keep the surface of your cornea smooth. If left untreated, dry eye disease can cause damage to the cornea and lead to vision loss.
Tears are composed of water, oil, and mucus. The oil in your tears prevents the water from evaporating, and the mucus acts to spread the tears evenly over the surface of your eye. When your tears don’t provide the necessary lubrication to protect them, you can develop the symptoms of dry eye disease.
Treatment for dry eye disease comes down to two main things: conserving the tears you produce and increasing the amount and/or quality of tears.
The simplest solution for mild cases of dry eye is over-the-counter artificial tears which can be found at any pharmacy and most supermarkets. We recommend only using artificial tears that do not contain any preservatives, as they can further irritate your eyes. If supplemental tears aren’t working for you, prescription eye drops may be an option to stimulate the natural production of tears. Some studies suggest that omega-3 fatty acids may help your body produce more quality tears.
In some cases, tears drain from the eyes too quickly. Treatment to conserve tears can be achieved through the use of tiny plugs which are inserted into your tear ducts. In healthy eyes, tear ducts regulate the volume of tears in your eyes by draining excess fluid into a cavity that leads to the back of your nose. If you’re not producing enough tears, plugging these ducts will allow your eyes to retain tears longer.
Since the cornea does not contain blood vessels, it is more prone to infections than the rest of the eye. Common eye infections, such as conjunctivitis (pink eye), can cause swelling, discharge, redness, itching, burning, and hindered vision. The cornea can also be infected by herpes simplex with similar symptoms to conjunctivitis.
Treatment for eye infection is usually as simple as eye drops or a topical ointment. If it turns out to be a bacterial infection, antibiotics may be prescribed. Eye infections should be treated as soon as possible. If left untreated they can lead to permanent damage to the eye and even vision loss.
Shingles (Herpes Zoster)
Shingles is an extremely painful condition caused by the same virus as chickenpox. The virus lays dormant in the nervous system for years and can eventually reactivate due to advanced age or a weakened immune system. If the shingles infection is on the face, it can affect the cornea. Cornea damage happens after the shingles infection has appeared to clear up, so be sure to see an ophthalmologist if you’ve had shingles affect your face.
Corneal dystrophies are typically congenital conditions that often develop gradually and lead to vision loss. Changes in the shape and clarity of the cornea are caused by corneal dystrophies and can have a wide range of effects. There are more than 20 varieties of corneal dystrophies and the symptoms for each are different, as well as how long they can last.
Treatment for corneal dystrophies ranges as much as the many different forms. Some can be easily treated simply with eye drops, while others need a corneal transplant or PTK laser treatment. Comprehensive eye exams include checking for corneal dystrophy. Early detection can improve the results of treatment.